The Return of the Third-Grade Ghosthunters by Grace Maccarone, 1989.
This is part of the Third Grade Ghosthunters series.
The same kids who were assigned to figure out the mysterious happenings at their school are now going on a field trip to a farm with their class. They’ll be spending a couple of days on the farm, sleeping over. Adam has been looking forward to the trip, and he brings his ghost hunting gear with him, just in case. He hasn’t lost his taste for ghost stories since their last adventure.
At first, things are pretty normal. Then, during the night, the boys hear howling and see apparitions of a ghostly wolf in their bunk house. Could the farm be haunted? Or is this some kind of mean joke? Perhaps it’s the work of the class bully, Jeff? The ghost hunting committee assembles again to solve the mystery.
They try to use scientific methods to study the haunting. They measure the placement of the furniture to see if anything moves while they’re out and sprinkle flour on the floor to get footprints. But, even though they watch carefully and don’t see anyone enter the bunk house, when they go inside, everything is a mess, and the flour doesn’t appear disturbed. Everyone else is about ready to admit that this one is a real ghost, but Adam has another theory.
The solution has an interesting twist to it. It’s a good mystery for kids beginning chapter books who like a little creepiness in their reading.